Forming a film of a large-area semiconductor or the like at a high deposition rate at a reduced cost is essential to the improvement of productivity of the production line and the reduction of cost in manufacturing semiconductor devices including photovoltaic devices, optical sensors, electrophotographic photoconductors and liquid crystal driving circuits.
Plasma CVD processes are the most general and preferable processes of forming a large-area semiconductor film by deposition. The plasma CVD process is to decompose a raw material gas to produce plasma causing the formation of a deposited film on a substrate.
Among various plasma CVD processes, glow discharge decomposition process (hereinafter referred to as "GD process") has most widely been used because of its satisfactory plasma control performance and capability of comparatively easily forming a large-area film In the GD process, high RF waves are applied to a raw material gas to decompose the raw material gas in plasma state and the deposition of a film is caused on a substrate. However, the deposition rate of the GD process is not sufficiently high; for example, the deposition rate of the GD process is on the order of 20 Angstrom/sec at the maximum in depositing a hydrogenated amorphous silicon film (hereinafter referred to as "a-Si:H film"). And the GD process is accompanied with a problem that when the supply power is increased to enhance the deposition rate beyond the foregoing limit, in most cases, the quality of the film deteriorates sharply with the increase of the supply power Furthermore, increase in the supply power accelerates the vapor phase reaction excessive, entailing the deposition of a large amount of powdery substances on surfaces, such as the walls of the film forming chamber, other than the surface of the substrate. Such powdery substances leaking from the film forming chamber has the danger of burning and the possibility of falling on the substrate to form a defective film. Although dependent on the type of the raw material gas, and the shape and distance between the electrodes for applying a high RF voltage to the raw material gas, it is one of causes of such a problem that the reduction of the pressure of the gas below 0.1 torr in the GD process is difficult and hence the supply of large power is liable to accelerate the vapor phase reaction excessively. Furthermore, plasma density in the GD process is on the order of 10.sup.10 /cm.sup.3 at the highest because the plasma is intercepted.
Recently, the microwave plasma CVD process, which decomposes the raw material gas by microwave energy to produce a plasma of the raw material gas, causing the formation of a deposited film on a substrate, has been used increasingly. Since the microwave plasma CVD process uses microwaves of frequencies higher than those of the high-frequency waves employed in the GD process, discharge occurs at a comparatively low voltage and the density of the plasma is as high as 10.sup.12 /cm.sup.3.
In the microwave plasma CVD process, discharge can occur easily even if the pressure of the material gas is, for example, on the order of 10 millitorr, so that the vapor phase reaction is not accelerated excessively, and hence powdery substance is not deposited even if a large power is supplied. Consequently, a semiconductor film of a satisfactory quality can be formed at a high deposition rate. For example, the deposition rate of the microwave plasma CVD process in forming an a-Si:H film is 100 Angstrom/sec or higher.
In the microwave plasma CVD process, transmitting high-energy microwaves through a waveguide and a dielectric window into a film forming chamber is the most prevalent and practical. However, when high-energy microwaves are transmitted through the dielectric window, problems arises in this microwave transmitting method; that is, the decomposed material gas forms a film over the dielectric window, the film falls off the dielectric window onto a substrate on which a film is to be formed, forming defects in a film deposited over the substrate; the film adhering to the dielectric window is heated by the microwaves, cracking the dielectric window or the film formed over the dielectric window reduces the microwave transmittivity of the dielectric window, entailing variation in the deposition rate. These problems become intensified particularly when the energy of the microwaves is increased and the duration of film forming operation is extended.
In some cases, the interior of the film forming chamber is etched after completing the film forming operation to avoid the breakage of the dielectric window and to prevent the reduction of deposition rate, which, however, requires additional time increasing film-formation cycle time and there is the possibility of the components of the etching gas being mixed in a film deposited in the next film-formation cycle to deteriorate the quality of the deposited film.